doc: Fix syntax in ALTER FOREIGN DATA WRAPPER example

The example for dropping an option was incorrectly quoting the
option key thus making it a value turning the command into an
unqualified ADD operation. The result of dropping became adding
a new key/value pair instead:

 d=# alter foreign data wrapper f options (drop 'b');
 ALTER FOREIGN DATA WRAPPER
 d=# select fdwoptions from pg_foreign_data_wrapper where fdwname='f';
  fdwoptions
 ------------
  {drop=b}
 (1 row)

This has been incorrect for a long time so backpatch to all
supported branches.

Author: Tim <tim.needham2@gmail.com>
Discussion: https://postgr.es/m/170292280173.1876505.5204623074024041738@wrigleys.postgresql.org

Author: danielgustafsson

doc/src/sgml/ref/alter_foreign_data_wrapper.sgml

@@ -153,7 +153,7 @@ ALTER FOREIGN DATA WRAPPER <replaceable class="parameter">name</replaceable> REN
    Change a foreign-data wrapper <literal>dbi</literal>, add
    option <literal>foo</literal>, drop <literal>bar</literal>:
 <programlisting>
-ALTER FOREIGN DATA WRAPPER dbi OPTIONS (ADD foo '1', DROP 'bar');
+ALTER FOREIGN DATA WRAPPER dbi OPTIONS (ADD foo '1', DROP bar);
 </programlisting>
   </para>